Spectrometers are used to measure the properties of light produced by a particular source. For example, spectrometers can be used to measure the wavelength of incoming light from a source. Usually, this is done using a spectrograph, which is typically a spectrometer that has a detector attached, and which has a sensor that is able to detect peaks across a wavelength range. The detectors on a spectrograph are usually interchangeable, so that a range of detectors are able to be used.
There are a number of different configurations of spectrometers. For example, a Czerny-Turner spectrometer has a slit through which light from a wide bandwidth source may pass. The light diverges from the slit and is collimated and passed onto a disperser, which separates the light into its constituent wavelengths. The light then passes to a focusing element, which is used to focus the light onto an imaging or analysing plane where the sensor of a detector may be located. In this manner, the wavelength spectrum of a light source can be assessed.
Frequently, a light source will have a wide wavelength spectrum; so, each spectrometer is usually designed to be capable of use over a wide range of wavelengths. This also saves on costs as a single spectrometer can be used for a range of light sources that have a range of wavelength distributions. Therefore, typically, a spectrometer will be capable of use over a wavelength range from approximately 180 nm to 16 μm (i.e. 16 micrometres/microns). This means that, typically, mirrors will be used to collimate and to focus the light as, for such a large range in wavelengths, it is not possible to use lenses. This is because lenses are usually only designed for, and capable of, functioning over a relatively small range of wavelengths to ensure the optical performance needed.
In order to disperse the light, diffraction gratings are typically used as dispersers. It is common for a diffraction grating to be “blazed” at a particular angle, i.e. the diffraction efficiency is optimised at a particular wavelength range. Different groove spacings are available on gratings; a low groove density grating such as 75 grooves per millimetre (g/mm) will allow a relatively large portion of the EM spectrum to fall on the sensor, but with a much lower spectral resolution than would a 3600 g/mm grating that would direct a smaller portion of the spectrum to the detector.
Each grating can only be used up to a certain angle, due to geometric considerations, and so high groove density gratings cannot be used to analyse high wavelengths. For these reasons, it is often the case that a user wishes to swap easily between multiple gratings, either to analyse different wavelength ranges, or to analyse with higher resolution but a shorter wavelength range. Therefore, if multiple gratings are to be used, the grating in a spectrometer with which the light interacts will have to be exchanged for a different grating. This can, for example, be affected by removing one grating from the spectrometer and replacing it with another, or by having a number of gratings on a rotatable turntable and turning the turntable to move one grating to intercept the light in place of another.
To get the best results from a spectrometer, a detector must be located as close as possible to the focus point of the spectrometer. Typically, this is achieved by attaching the detector to the spectrometer with a flange so that the detectors sensor sits at the focus point. Once the detector has been fixed in place, any later adjustments are usually difficult as the spectrometer and the detector are heavy, difficult to move and are sensitive instruments, so disruption is not a trivial matter and may take significant time and effort.
Although it is important that the detector is located at the focal point of the focusing element, it may not be possible to know exactly where the focus point is. As such, the spectrometer is calibrated to have the focal point located at an analysis plane, which is the position at which the sensor of a detector is positioned when the detector is attached to the spectrometer. When the focus point is in this position, the spectrometer is “in focus”. However, a number of factors can cause the focal point to shift causing the spectrometer to be out of focus.
There are a number of reasons that the focal point may shift. For example, the angle at which the diffraction grating is presented to the focusing element alters optical aberrations present at the detector that can be mitigated by a change in focus. Spectrometers may have more than one entrance option, and more than one exit option; these optional light paths are utilised by the insertion of a plane mirror at the appropriate point. The change in configuration may bring about a small change in focus. Additionally, no two diffraction gratings will be the same, and their surface texture (e.g. surface flatness) will be different. Therefore, although the difference in surface flatness may be as small as a few microns, this can introduce an additional ‘optical power’ (i.e. the degree to which the system converges or diverges the light) to the system which can, again, be mitigated by the position of the focusing element. The shift in the position of the focus point is likely to be small, therefore requiring small and precise adjustments to the position of the detector in order to restore it to the position at which the spectrometer is focused.
Making precision adjustments to the position of a detector causes significant difficulties due to the size of the detector and the means by which it is attached to the detector (i.e. the flange). Indeed, due to the precision needed for the adjustments, it can be necessary to completely remove the detector from the spectrometer to enable the flange and connections to be appropriately adapted before the spectrometer can be replaced in the correct position. This is impractical.
Therefore, a problem exists as to how to ensure that the focus of the spectrometer is maintained for different configurations and a wide range of measurements, without manually intervening in the delicate optical set-up of the spectrometer.